We examined oral glucose tolerance, plasma lipid/lipoprotein levels, and whole body leucine kinetics in very old physically frail men and women, and compared them to the same parameters measured previously in 60-70 yr. old healthy men and women to assess whether advancing age and reduced physical activity alter substrate metabolism. Whole body protein kinetics were evaluated using a constant intravenous 13C-leucine infusion in 4 frail women and compared to four 60-67 yr old women . Lower rates (P<0.05) of fasting whole body protein breakdown , protein synthesis and leucine oxidation were observed in the frail women than in the 60-67 yr. old women, suggesting that whole body protein turnover is reduced in physically frail women, perhaps to conserve nitrogen and protect against protein wasting.